Add CLZ opcode (EIP-7939)

[mw2000]

Summary

Add the Count Leading Zeros opcode (CLZ, 0x1E). Activates with Fusaka.

Spec

• EIP-7939: Count Leading Zeros (CLZ) Opcode — https://eips.ethereum.org/EIPS/eip-7939

Scope

• Add Fusaka hardfork + EIP-7939 flag to EEVM.HardforkConfig.
• Implement the CLZ opcode in the interpreter (single 256-bit input → leading-zero count, 0..256).
• Charge G_low (5 gas).
• Gate the opcode on the flag — earlier hardforks must reject it as invalid.

Definition of Done

• CLZ(0) returns 256, CLZ(1) returns 255, CLZ(2^255) returns 0.
• Fusaka CLZ StateTest fixtures pass.

Parent

• EVM Completeness: Missing opcodes, precompiles, and gas rules #30 — EVM Completeness tracking issue